(71c) Modeling Mesoscale Structure in Comb Polymer Materials for Proton Transport Applications

The industry standard for proton exchange (or polymer electrolyte) membranes for fuel cells is Nafion. Nafion is an effective proton conductor in its hydrated state but loses this capability at high temperatures when the degree of hydration is greatly reduced. It is naturally worthwhile to investigate whether there are materials that can achieve high rates of proton transport in the anhydrous state and this has been the subject of several recent investigations. In particular, a recent study by our colleagues on the properties of a series of N-heterocycle-functionalized comb polymers indicated that some of these polymers exhibit substantially elevated proton conductance. Moreover structural characterization of the materials provided evidence of mesoscale structure that might facilitate that transport via confinement of the proton conducting functionalities within cylindrical or lamellar mesophases. These results highlight the potential for making anhydrous proton conducting materials that exploit the mesoscale structural characteristics of branched polymers. The purpose of the present paper is to provide additional insights into these observations from Monte Carlo simulation studies of coarse-grained models of these comb polymers in which we determine the mesoscale structure and relate it to the polymer architecture. We have constructed simple coarse-grained models of the comb polymers that incorporate the chain architecture but require only a single chi-parameter for interchain interactions. We have studied these models using Monte Carlo simulations using the single chain in mean field (SCMF) method developed by Muller, de Pablo and their coworkers. We find that this method allows us to study the spontaneous self-assembly of the model comb polymers into mesophase structures with both lamellar and cylindrical symmetry consistent with those seen in the experiments. Our calculations also suggest an explanation for the disordered morphologies found for some of the polymers.